Properties and Overview of Rhodium

Overview:

Rhodium (Rh) is a rare, silvery-white metal with the atomic number 45 and the symbol Rh. It belongs to the platinum group of metals (PGMs) in Group 9 of the periodic table and is known for its remarkable resistance to corrosion, high reflectance, and hardness. Discovered in 1803 by English chemist William Hyde Wollaston, rhodium was isolated from a platinum ore that also contained palladium. The name "rhodium" is derived from the Greek word "rhodon," meaning "rose," due to the rose-colored compounds formed by some of its salts. Rhodium is one of the rarest elements in the Earth's crust, with an average concentration of about 0.001 parts per million. It is typically found in ores mixed with other PGMs, particularly in nickel and platinum deposits. Physically, rhodium is a hard, durable metal with a high melting point of 1,964°C and a boiling point of about 3,695°C. Its density is 12.41g/cm³, making it less dense than platinum but still quite dense compared to most metals. Rhodium is highly reflective and retains its luster even at high temperatures, so it is often used for surface coatings and plating. The metal is also highly resistant to corrosion and oxidation, even at elevated temperatures, which makes it ideal for applications requiring a combination of durability and aesthetic appeal. Rhodium's crystalline structure is face-centered cubic, a common structure for metals that contributes to its hardness and resistance to deformation.
Chemically, rhodium is relatively inert and does not readily react with most substances, characteristic of noble metals. It is resistant to attack by acids, including nitric acid and hydrochloric acid, although it can dissolve in hot, concentrated sulfuric acid. In its most common oxidation state of +3, rhodium forms a variety of compounds, including rhodium(III) chloride (RhCl₃), which is used as a precursor to many rhodium catalysts. Rhodium can also exist in other oxidation states, such as +1, +2, +4, and +5, but these are less stable and less commonly encountered. Rhodium compounds are typically used in catalytic applications, where the metal's ability to facilitate chemical reactions without undergoing permanent change is highly valuable. As a catalyst, rhodium is particularly effective in promoting hydrogenation, hydroformylation, and oxidation reactions.
In terms of safety, metallic rhodium is considered relatively safe for handling under normal conditions. However, care must be taken with rhodium compounds, especially those used as catalysts, as they can be toxic if ingested, inhaled, or absorbed through the skin. Rhodium dust and powder can pose respiratory hazards if inhaled. Therefore, appropriate safety measures, such as using protective gloves, masks, and proper ventilation, should be taken when handling it in industrial or laboratory settings. Despite its rarity and high cost, rhodium is not classified as a significant toxic hazard, but like many metals, it can cause allergic reactions or dermatitis upon prolonged exposure.

Production:

Rhodium is primarily produced by mining and refining platinum and nickel ores. Most of the world's rhodium supply is obtained from key mining regions, including South Africa, Russia, and Canada. The extraction process typically involves separating rhodium from other platinum group metals through chemical reactions and purification steps, including precipitation, solvent extraction, and refining by electrolysis or other chemical means. The final product is typically a pure metallic powder or ingot, which can be further processed into various forms for industrial use.

Applications:

The applications of rhodium are diverse, primarily due to its excellent catalytic properties, corrosion resistance, and high reflectivity. The most extensive use of rhodium is in catalytic converters for automobiles, where it catalyzes to reduce harmful emissions by converting nitrogen oxides (NOx) into nitrogen and oxygen, as well as carbon monoxide and hydrocarbons into carbon dioxide and water. This application alone accounts for the majority of global rhodium consumption. In addition to its use in catalytic converters, rhodium is also employed in various industrial processes that require catalysts, including the production of acetic acid, hydrogenation of organic compounds, and the oxidation of ammonia in nitric acid production.
Rhodium is also used in the jewelry industry, where it is applied as a thin electroplated coating to enhance the luster and durability of silver, white gold, and platinum jewelry. The bright, reflective surface of rhodium plating provides a protective layer that resists tarnishing and scratching, making it highly desirable for high-end jewelry pieces. In addition, rhodium's ability to reflect light makes it valuable for optical instruments and mirrors, especially in high-temperature and corrosive environments.
In the electronics industry, rhodium is used for its conductivity and corrosion resistance, particularly in electrical contacts and connectors requiring high reliability and longevity. Due to its stability and resistance to oxidation, it is also employed in thermocouples for high-temperature measurements.

Summary:

Rhodium is a rare and valuable noble metal known for its exceptional catalytic properties, resistance to corrosion, and reflective qualities. It is used extensively in automotive catalytic converters, industrial catalysts, jewelry plating, and electrical components. While relatively non-toxic, safety measures are necessary when handling rhodium compounds and powders. The production of rhodium is limited to a few key regions, and its high cost reflects its rarity and the complex processes required for its extraction and refinement. Despite these challenges, rhodium remains critical in various high-performance and high-value applications.

See a comprehensive list of atomic, electrical, mechanical, physical and thermal properties for rhodium below: